I have been doing general repairs, including lots of plumbing and septic, here in Central Vermont for over 20 years, so I am quite experienced. I have a client that bought a house last April. Two story chalet, slab on grade, built in the 1960s, 2nd home used on weekends (family of 4). They started noticing a septic smell, limited to a utility closet, starting about 6 weeks ago when they started a fire in the fireplace. The smell is only coming from this one closet that holds a sealed combustion, wall hung boiler, the pressure tank and the stacked washer/dryer. The closet is under the center staircase and is off the master bedroom and is centered on the south wall of the house. The only DWV piping in the closet is a drain for the washing machine that goes down into the slab (with water in the trap), and an 3 ABS cleanout in the floor. There are two full bathrooms on the NW corner of the ground floor. There is a hole in the slab below the bathtub for its trap (stone & plastic VB are visible below slab)- there is never a smell here or in either bathroom. The kitchen sink (on second floor) drain enters the soil stack above the west bathroom, almost at the west wall. Therefore, there is no other DWV piping above the slab within at least 10 feet of this closet (again, the only place with any septic smell). The only propane piping is for the boiler, and that comes right through the outside wall to the boiler. The second stage regulator is on the other side of this wall, outside. I carefully checked all of the joints, inside and outside, of the propane piping and regulator with my Bacharach Leakator Jr. and found nothing. It is not propane. I depressurized the house with a blower door to -50 pascals with respect to outside, and did not smell any septic, until we flushed the toilets and dumped the lav sinks all at the same time. The smell took a couple of minutes to appear, but it was only in that closet under the stairs. We repeated this later, without the blower door, and got the same smell after dumping water. When we did get the smell, my Leakator Jr. would start ticking only at the base of the stem wall in that closet. I tried sealing the slab/stem wall joint but we still get the smell from time to time, especially after dumping a bunch of water (such as this past weekend after several showers in a row). I had a local septic company check the pipes for cracks with a video scope (the same company that pumped the septic tank last April and also had inspected both IN and OUT pipes to tank. This time, they checked the pipe from the floor cleanout to the tank and from the cleanout to where it started branching to various fixtures (again, this is all piping under the slab). The only question was the condition of the cast iron pipe to the tank (which would be about 45 years old) and the condition of the joint between that cast iron pipe and the ABS building drain that connects to it. This is the only possible scenario that I can think of- a leak in the cast iron or the cast iron/ABS joint that is allowing septic gases to flow through what must be sandy soil to the house. We also ruled out any floor drains. The owner was present when the carpet was laid and there were no floor drains in the slab. We have clearly ruled out a dead mouse or anilmal. I did a smoke test of the piping (closing off the top of the soil stack) and found nothing. Short of digging up and replacing the building drain, which would be difficult to do now with a few feet of snow on the ground and temperatures in the single digits, does anyone have any other ideas on what the source may be or how to find the source?